Given all that raping, looting and massacring that went around, there'd be certain difficulties in portraying the guy in anything approaching positive light to modern audiences. And spectacle movie makers tend not be overly keen on real anti-heroes.

There may also be the issue that on the whole he was too overwhelmingly victorious, and to boot didn't die dramatically or anything. And, of course, the bit about burying him with gazillion concubines and putting to sword all the workers involved with building the grave, and reputedly killing everyone the funeral procession encountered in order to keep the location secret, is going to be a little difficult to put right...

Fact is, the Mongols weren't a very nice bunch.